Parachuting is a dangerous activity. If the primary parachute fails to deploy either due to malfunction or incapacity of the jumper, the reserve chute must be deployed with enough time to reduce the jumper's downward velocity to safe levels for ground contact. If the primary does not properly deploy and the reserve either does not deploy or does not deploy early enough, the jump will be fatal.
Recreational jumpers generally jump from a high altitude (nominally 5000 ft. above ground) to get the maximum flying time. Automated activations devices (AADs) like those made commercially by Airtec and Astra are used by jumpers in training to safely deploy the reserve chute if the inexperienced jumper goes too low before pulling the main (or if the main is inoperative or the jumper is incapacitated). These conventional AADs detect the jumper's altitude using air pressure above sea level either mechanically or electronically. When the altitude detected is about 1000 ft. above the ground level (these devices must be calibrated at ground level to set how high this is above sea level), these devices activate an automated actuation that deploys the reserve. U.S. Pat. No. 5,222,697 to Allen and U.S. Pat. No. 3,992,999 to Chevrier, et al. show typical actuation systems.
Because there is a relatively long period for safe determination that the main cute has not been deployed in recreational parachuting, current AADs use relatively in accurate pressure sensing to determine the correct time for reserve chute deployment. U.S. Pat. No. 4,858,856 to Cloth and U.S. Pat. No. 4,865,273 to Jones describe a purely mechanical pressure (i.e. altitude) detection systems that activate chute deployment. U.S. Pat. No. 5,825,667 to Van Den Broek describes a device the includes a data processing and an electronic pressure sensor that is used to determine height and an acceleration sensor that is used to compute a redundant height parameter to improve height determination accuracy if the jumper is oriented in the proper way (as detected by a tilt sensor). U.S. Pat. No. 6,378,808 to Smolders also uses an electronic means or measuring altitude as the prime determiner of parachute release time, but does the computation through a complex table driven model computed by an electronic processor. The currently available commercial AADs like the Astra or the Cypress (from Airtec) also uses barometric pressure and pressure changes to determine altitude and rate of decent.
The assumption in all of these prior AAD systems is that pressure change (i.e. altitude based on pressure adjusted for the pressure at ground level) is accurate enough for determination when to deploy the reserve chute (or in the case of an automated chute deployment the main chute). U.S. Pat. No. 5,825,667 acknowledges the accuracy limitations of this approach and incorporates a one axis accelerometer that can be used to improve this height estimate if the tilt sensor indicate that the jumper is in the proper downward facing orientation, but in all of these prior approach there is an assumption that accurate kinematics determination sensing is expensive and therefore should not be used or used in limited ways.
With the advent of low cost position, velocity, and acceleration measurement devices this assumption is no longer valid. While pressure sensing is still an important component to AAD operation, it can now be augment by partial or completely solved jumper kinematic parameter measurement starting at the point of departure from the aircraft down to the point of ground contact. All relevant function and failure mode conditions can be determined and used to trigger reliable reserve (or primary) chute deployment based on both kinematic parameters and pressure-derived altitude measurements.
The addition of direct kinematic measurement to the deployment decision is critical for determination of primary chute failure is low altitude military jumps within approximately the first 5 seconds of the jump and for accurate deployment of the main for precision airdrop.